Chemical ozone loss in the Southern Hemisphere lower stratosphere during winter and spring 1996 is investigated using a three-dimensional chemical transport model. Model diagnostic quantities are developed to determine the underlying chemical mechanisms controlling the distribution of ozone. These diagnostic tracers quantify the ozone change due to individual catalytic cycles or reaction mechanisms that form or destroy ozone. Results from these tracers confirm the importance of halogens not only in the development of the ozone hole but also in midlatitude ozone depletion of which almost 50% is due to cycles involving halogens. The dominant chemical ozone loss mechanism in the middle latitudes is due to the cycle whose rate determining step involves the bimolecular collision between HO2 and O3. Observational evidence from ozonesonde measurements shows substantial ozone depletion in the polar subvortex region (altitudes below ~14 km). These measurements also indicate that this ozone depletion must be due, at least in part, to in situ chemical ozone loss. Model results show that over a quarter of the ozone depletion associated with the ozone hole resides at subvortex altitudes. Diagnostic tracers designed to quantify ozone loss occurring above and below the subvortex transition altitude show that the majority of this ozone depletion is in situ ozone loss. Furthermore, at altitudes below the subvortex transition, polar ozone loss is efficiently transported to middle latitudes, where it contributes to midlatitude ozone decline.